|Material|Formula|Refractive index(es) 589.3 nm|Dispersion 431 – 687 nm|Hardness (Mohs' scale)|Density (g/cm3)|Thermal Cond.|State of the art|
|---|---|---|---|---|---|---|---|
|Diamond|C|2.417|0.044|10|3.52|Excellent|(Natural)|
|Glasses|Silica with Pb, Al, &/or Tl|~ 1.6|> 0.020|< 6|2.4 – 4.2|Poor|1700 –|
|White Sapphire|Al2O3|1.762 – 1.770|0.018|9|3.97|Poor|1900–1947|
|Spinel|MgO·Al2O3|1.727|0.020|8|~ 3.6|Poor|1920–1947|
|Rutile|TiO2|2.62 – 2.9|0.33|~ 6|4.25|Poor|1947–1955|
|Strontium titanate|SrTiO3|2.41|0.19|5.5|5.13|Poor|1955–1970|
|YAG|Y3Al5O12|1.83|0.028|8.25|4.55 – 4.65|Poor|1970–1975|
|GGG|Gd3Ga5O12|1.97|0.045|7|7.02|Poor|1973–1975|
|Cubic Zirconia|ZrO2(+ rare earths)|~ 2.2|~ 0.06|~ 8.3|~ 5.7|Poor|1976 –|
|Moissanite|SiC|2.648 – 2.691|0.104|8.5–9.25|3.2|High|1998 –|
|Quartz|Silica|1.543 – 1.554||7-|2.50 – 2.65||Ancient|

Question: how hard is a diamond?
Answer: 10